1. Field of the Invention
This invention relates to amplifiers, and more specifically, to a video amplifier.
2. Description of the Prior Art
In a video display driver, a video output amplifier provides a high-level video output signal to an image display device such as a cathode ray tube (CRT). The video output signal is conveyed to a signal input electrode of the CRT, e.g., a cathode electrode. Unfortunately, the CRT-signal input electrode can exhibit a capacitance large enough to impair the high-frequency response of the video signal developed at the output of the driver amplifier. Thus, it is desirable to reduce the effect of the CRT capacitance upon the driver-amplifier output signal, thereby preventing degraded high-frequency response and its associated degradation of image resolution.
FIG. 1 shows a prior art video output circuit 100 coupled to a signal input electrode of a CRT 105. Video output circuit 100 is used to reduce the effect of the capacitance of CRT 105 (represented by capacitor 110) upon driver-amplifier output signals. A typical value for capacitor 110 is 15 pF.
An input node 120 receives a video signal from a video input stage (not shown). The first buffer stage includes an NPN emitter-follower transistor 130 and a diode-connected PNP transistor 140, and the second buffer stage includes a PNP emitter-follower transistor 150 and a diode-connected NPN transistor 160. The voltage supplies are the positive supply voltage VCC and ground.
NPN emitter-follower transistor 130 and diode-coupled PNP transistor 160 are arranged in cascade in the output signal path from input node 120 to capacitance 110. Similarly, PNP emitter-follower transistor 140 and diode-coupled NPN transistor 160 are arranged in cascade in the output signal path from input node 120 to capacitance 110. Diode-coupled transistors 140 and 160 are connected across the base-emitter junctions of transistors 130 and 150, respectively. In addition, diode-coupled transistors 140 and 160 and are biased for forward current conduction in a direction opposite to the forward current conduction of the associated base-emitter junctions of transistors 130 and 150, respectively.
A rapid positive-amplitude transition on input node 120 is propagated to CRT 105 via conductive transistor 130 and conductive transistor 160. Transistors 140 and 150 are reversed biased (i.e. nonconductive) at this time. Conversely, rapid negative amplitude transitions on input node 120 are propagated to CRT 105 via conductive transistors 140 and 150. Transistors 130 and 160 are reversed biased at this time.
Output circuit 100 has a disadvantage in that for either positive or negative amplitude transitions, only one stage of current gain is provided because diode-connected transistors 140 and 160 provide no current gain. Moreover, because current gain is limited, output circuit 100 must be provided with a significant input current at input node 120 to drive the output capacitance, especially at relatively high frequencies. This need for a high input current translates into a need for an input stage with excessive power requirements.
For the foregoing reasons, there is a need for a video amplifier that includes a high-bandwidth, low-power voltage gain stage coupled to a high-current-gain output stage.